Generally, transmissions for hybrid vehicles use a system in which two motor generators, that is, a first motor generator (hereinafter, referred to as ‘MG1’) and a second motor generator (hereinafter, referred to as ‘MG2’), and an engine are connected to a planetary gear set so that continuously variable shifting can be automatically conducted by controlling the MG1 and the MG2.
Particularly, in a step type automatic transmission system using planetary gears, each gear stage has a specific gear ratio (an input speed/an output speed), and gears are automatically shifted between gear stages. To embody the gear stages, hydraulic pressure of a clutch or friction elements, i.e., a break mechanism, are controlled. Gear shifting is automatically conducted by controlling hydraulic pressures of the friction elements and engaging or releasing the friction elements or performing the engagement and release of the friction elements at the same time.
In the conventional step type automatic transmission system, the input speed and the output speed of the transmission form a predetermined speed ratio in response to a corresponding gear shift ratio. While a gear shift ratio is converted into another gear shift ratio, the output speed is constant, but the input speed is varied to convert the gear shift ratio from the existing ratio to a subsequent ratio. Therefore, the conventional automatic transmission system uses only the input speed of the transmission as a control factor when shifting gears and controls engagement and release of the corresponding friction elements.
A related conventional art was proposed in Korean Patent Registration No. 10-0969366 (Publication date: Jul. 9, 2010, hereinafter referred to as ‘patent document 1’), entitled “METHOD FOR CONTROLLING GEAR SHIFTING OF AUTOMATIC TRANSMISSION”. This conventional art provides a technique for controlling gear shifting in such a way as to control the engagement and release of friction elements when skip shifting of the automatic transmission that skips three gear ratio steps from a 6th gear stage to a 3rd gear stage is conducted.
FIG. 1 is a block diagram showing a system for conducting a method for controlling gear shilling of an automatic transmission according to patent document 1.
As shown in FIG. 1, the system for conducting the gear shifting control method according to patent document 1 includes a throttle opening degree sensor 110, a vehicle speed sensor 120, a turbine rpm sensor 130, a hydraulic pressure sensor 140, a transmission control unit 150 and a hydraulic pressure control unit 170. In the technique for controlling gear shifting of the automatic transmission according to patent document 1, so as to shift gears from a current N gear stage in which the vehicle is traveling to an N-3 gear stage, the transmission control unit 150 uses only the rpm of a turbine which is sensed from the turbine rpm sensor 130. In other words, it uses only one control factor to control engagement and release of the friction elements. Therefore, precise control for engagement and release of the friction elements when shifting gears is limited. In the gear shifting control method according to patent document 1, engagement and release of the friction elements are consecutively conducted rather than being performed at the same time. Thus, it is difficult to simultaneously and individually control hydraulic pressures applied to the friction elements.
Particularly, in the case of automatic gear shifting using an electronically controlled continuously variable transmission (ECVT), because a transmission input speed and a transmission outlet speed of each gear stage may vary rather than being at a constant gear ratio, if only the transmission inlet speed is used as a control factor, it becomes impossible to control the engagement and release of the friction elements when the gears are shifted.